IRI ENSO Forecast
IRI Technical ENSO Update
Published: October 18, 2019
Note: The SST anomalies cited below refer to the OISSTv2 SST data set, and not ERSSTv4. OISSTv2 is often used for real-time analysis and model initialization, while ERSSTv4 is used for retrospective official ENSO diagnosis because it is more homogeneous over time, allowing for more accurate comparisons among ENSO events that are years apart. During ENSO events, OISSTv2 often shows stronger anomalies than ERSSTv4, and during very strong events the two datasets may differ by as much as 0.5 C. Additionally, the ERSSTv4 may tend to be cooler than OISSTv2, because ERSSTv4 is expressed relative to a base period that is updated every 5 years, while the base period of OISSTv2 is updated every 10 years and so, half of the time, is based on a slightly older period and does not account as much for the slow warming trend in the tropical Pacific SST.
Recent and Current Conditions
In mid-October 2019, warmish but neutral ENSO SST conditions were observed in the NINO3.4 region. The September SST anomaly was -0.02 C, very close to average, and for Jul-Sep it was 0.18 C, also in the neutral range. The IRI’s definition of El Niño, like NOAA/Climate Prediction Center’s, requires that the SST anomaly in the Nino3.4 region (5S-5N; 170W-120W) exceed 0.5 C. Similarly, for La Niña, the anomaly must be -0.5 C or less. The climatological probabilities for La Niña, neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season. The most recent weekly anomaly in the Nino3.4 region was 0.4 C, in the upper portion of the ENSO-neutral range. SST anomalies continue to be near zero in the eastern equatorial Pacific, slightly positive in the central Pacific and more strongly positive in the west-central and western part of the basin. Some atmospheric variables have leaned somewhat toward El Niño in the last month, such as the low-level and upper-level zonal wind anomalies and the Southern Oscillation index. However, these variables may reflect intraseasonal variability more than the ENSO condition. The anomalous convection pattern reflected ENSO-neutral conditions, with suppressed convection both near the dateline and over Indonesia. Subsurface temperature anomalies from the dateline eastward in the equatorial Pacific became somewhat positive during the last month, with strongest positive anomalies in the central Pacific while somewhat negative anomalies continued at depth farther east of the dateline.
Expected Conditions
What is the outlook for the ENSO status going forward? The most recent official diagnosis and outlook was issued approximately one week ago in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it states that the currently neutral conditions are most likely to continue through the fall, winter and spring. The latest set of model ENSO predictions, from mid-October, now available in the IRI/CPC ENSO prediction plume, is next discussed: As of mid-October, 15% of the dynamical or statistical models predict El Niño conditions for the Oct-Dec season, while 85% predict ENSO-neutral and 1% predict La Niña. Going forward, these large probability differences diminish only slightly, and in most of the seasons from Nov-Jan through Jun-Aug 2020 about 70-81% predict neutral, 15-21% predict El Niño and 3-7% predict La Niña.
Note – Only models that produce a new ENSO prediction every month are included in the above statement.
Caution is advised in interpreting the distribution of model predictions as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.
An alternative way to assess the probabilities of the three possible ENSO conditions is more quantitatively precise and less vulnerable to sampling errors than the categorical tallying method used above. This alternative method uses the mean of the predictions of all models on the plume, equally weighted, and constructs a standard error function centered on that mean. The standard error is Gaussian in shape, and has its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill results in a relatively narrower error distribution, while low skill results in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña 10% or less for all forecast seasons except for the last season, Jun-Aug, when the probability rises to 16%. Probabilities for neutral conditions begin at 74% for Oct-Dec, then drop just slightly to 65-70% for Nov-Jan through Apr-Jun, and drop further to 50-60% for May-Jul and Jun-Aug. Probabilities for El Niño begin at 25% for Oct-Dec and remain in the 25-30% range from Nov-Jan through Apr-Jun, then finally rise to 30-35% for May-Jul and Jun-Aug 2020. A plot of the probabilities generated from this most recent IRI/CPC ENSO prediction plume using the multi-model mean and the Gaussian standard error method summarizes the model consensus out to about 10 months into the future.
The same cautions mentioned above for the distributional count of model predictions apply to this Gaussian standard error method of inferring probabilities, due to differing model biases and skills. In particular, this approach considers only the mean of the predictions, and not the total range across the models, nor the ensemble range within individual models.
In summary, the probabilities derived from the models on the IRI/CPC plume describe, on average, a marked tilt of the odds toward neutral ENSO conditions for the entire forecast period of Oct-Dec through Jun-Aug 2020, with El Niño more likely (25-35%) than La Niña (1-16%) from Nov-Jan onward to the final forecast season of Jun-Aug which is the only season with La Niña probability exceeding 10%. A caution regarding this latest set of model-based ENSO plume predictions, is that factors such as known specific model biases and recent changes that the models may have missed will be taken into account in the next official outlook to be generated and issued early next month by CPC and IRI, which will include some human judgment in combination with the model guidance.
IRI/CPC Mid-Month Model-Based ENSO Forecast Probabilities
| Season |
La Niña |
Neutral |
El Niño |
| OND 2019 |
1% |
74% |
25% |
| NDJ 2019 |
3% |
68% |
29% |
| DJF 2019 |
5% |
67% |
28% |
| JFM 2019 |
4% |
69% |
27% |
| FMA 2020 |
3% |
71% |
26% |
| MAM 2020 |
2% |
72% |
26% |
| AMJ 2020 |
5% |
68% |
27% |
| MJJ 2020 |
10% |
57% |
33% |
| JJA 2020 |
16% |
53% |
31% |
